This invention relates to the method of recovery of caprolactam from nylon 6 oligomers, more particularly from a concentrated nylon 6 chip wash water containing water, cyclic oligomer, and caprolactam.
Nylon 6 can be prepared in a continuous flow through a series of kettles, such as in U.S. Pat. No. 3,294,756, hereby incorporated by reference. The polymer resulting from this continuous kettle process is formed into pellets or chips of solid polymer containing water-extractable impurities which can be washed or extracted out of the chips by water washing, such as in U.S. Pat. Nos. 3,245,964 and 3,373,145, both hereby incorporated by reference. The washed polymer chips are then dryed, as in U.S. Pat. No. 3,266,165 hereby incorporated by reference, and are ready to be further processed such as by being fed to an extruder feeding a spinning system for spinning the polymer into fiber or yarn by conventional known methods. The resulting fiber is useful as tire and industrial yarn and as carpet or apparel yarn.
The water used to wash nylon chips contains valuable components extracted from the polymer chip such as caprolactam and cyclic lactam oligomers described in U.S. Pat. No. 3,245,964 above. Previously, only the water extracted monomer, i.e., caprolactam, was recovered from the wash water. The caprolactam was recovered by multiple distillation steps followed by multiple evaporation steps and finally crystallization of the caprolactam. The cyclic oligomer was wasted in solid residues from the bottom of the distillation and evaporation vessels.
Also, it is known to depolymerize scrap nylon, in the form of chips, yarn and the like with phosphoric acid or orthophosphoric acid in U.S. Pat. No. 3,182,055, hereby incorporated by reference.